The present invention relates to a synchronous belt for use as a timing belt in an automotive engine or the like.
Conventionally, synchronous belts have been manufactured of chloroprene rubber. However, as the demand on miniaturized automotive engine rooms increases, a higher heat resistance has been required of synchronous belts.
As disclosed in Japanese Laid-Open Patent No. 60-172749, there has been known a synchronous belt in which a hydrogenated nitrile rubber composition .obtained through sulfur vulcanization is used for the backing rubber, the tooth rubber, and the coating rubber on a facing fabric attached to the tooth rubber, so as to provide a higher heat resistance.
There has also been proposed a synchronous belt, as disclosed in Japanese Laid-Open Patent No. 1-87937 or the like, in which a rubber composition obtained through peroxide vulcanization with an organic peroxide serving as a vulcanizing agent is used for the backing rubber and the tooth rubber, while a hydrogenated nitrile rubber composition obtained through sulfur vulcanization is used for the coating rubber on the facing fabric.
However, the former synchronous belt in which the backing rubber is composed of the hydrogenated nitrile rubber composition obtained through sulfur vulcanization is still unsatisfactory in terms of heat resistance. To provide sufficient heat resistance, it can be considered to constitute each of the backing rubber, the tooth rubber, and the coating rubber on the facing fabric by the hydrogenated nitrile rubber composition. In this case, however, the above coating rubber cannot provide sufficiently strong adhesion between the facing fabric and the tooth rubber, resulting in early peeling off and breaking of the facing fabric.
To solve the above problem, the latter synchronous belt uses the hydrogenated nitrile rubber composition obtained through peroxide vulcanization in the backing rubber and in the tooth rubber, while using the rubber composition obtained through sulfur vulcanization in the coating rubber. However, since different vulcanizing agents are used in subjecting the synchronous belt to a molding/vulcanizing process under pressure, the sulfur in the coating rubber shifts toward the tooth rubber, while the organic peroxide in the tooth rubber shifts toward the coating rubber, so that a vulcanization density (crosslinking density) is easily reduced. As a result, the amount of the coating rubber abraded from the facing fabric becomes significant, which is limiting the extension of the durable life of the synchronous belt.